The PCI bus system is used primarily in the PC sector. In this case, the majority of PCs have both PCI slots and, for reasons of compatibility, ISA slots.
FIG. 1 shows the basic construction of a PCI bus system. Plug-in cards K1, K2, K3 are connected to the main circuit board or motherboard of the computer via the PCI bus. For this purpose, the plug-in cards K1, K2, K3 are inserted into the PCI slots. The power supply of the plug-in cards K1, K2, K3 is likewise effected via the PCI bus.
Older PCI plug-in cards K1, K2, K3 do not carry out power supply management or power management and are only supplied with a main supply voltage VCC. Such PCI plug-in cards are increasingly being replaced by plug-in cards which carry out power supply management in order to save energy. For this purpose, the PCI plug-in cards have to be supplied with a secondary supply voltage or auxiliary supply voltage Vaux via the PCI bus. The secondary supply voltage, having a small loading capability, supplies the PCI plug-in cards K1, K2, K3 in a standby mode or is used for the start-up of the computer by the PCI plug-in cards, the PCI plug-in cards being supplied with the main supply voltage VCC after start-up has been effected.
Since not all PCI bus systems are provided with a secondary voltage supply line for supplying the PCI plug-in cards with a secondary or auxiliary supply voltage Vaux, an interface circuit is provided on the PCI plug-in cards. The interface circuit ensures that the PCI plug-in cards are supplied with the main supply voltage VCC if no auxiliary supply voltage Vaux is present. Conversely, when an auxiliary supply voltage Vaux is present on the PCI bus, the PCI plug-in card K receives said auxiliary supply voltage for carrying out the power supply management.
FIG. 2 shows an interface circuit according to the prior art.
The interface circuit has two signal inputs E1, E2. The main supply voltage VCC is applied to the input E1, while the input E2 is connected to the auxiliary voltage supply line of the PCI bus. Furthermore, the interface circuit shown in FIG. 2 has an output A for outputting a supply voltage to a circuit situated on the PCI plug-in card. The circuit situated on the plug-in card detects the presence of an auxiliary supply voltage Vaux, applied to the input E2, via a detection line D. If no auxiliary supply voltage Vaux is present at the input E2, the detection device of the circuit situated on the PCI plug-in card is pulled to ground via a pull-down resistor R. As a result, the detection device receives a logically unambiguous signal indicating that no auxiliary supply voltage is present.
The interface circuit according to the prior art, as is shown in FIG. 2, contains two switching devices S1, S2, which are formed by two complementary transistors in the example shown in FIG. 2. In this case, the transistor S1 is an N-channel FET, while the transistor S2 is a P-channel FET. The control terminals of the two transistors S1, S2 are connected to the input E2. If the auxiliary supply voltage Vaux is present at the input E2, the FET transistor S1 is turned on or activated, and, at the same time, the FET transistor S2 is turned off or deactivated. As a result, the auxiliary supply voltage for the PCI plug-in card is present at the output A. At the same time, the auxiliary supply voltage Vaux present at the input E2 is detected by a voltage detection device, present on the PCI plug-in card, via the signal output D of the interface circuit.
If the main supply voltage VCC is present at the input E1 and if, at the same time, no auxiliary supply voltage Vaux is applied to the input E2, the FET transistor S1 turns off and the complementary FET transistor S2 is turned on, with the result that the plug-in card circuit is supplied with the main supply voltage VCC via the output A of the interface circuit.
If the main supply voltage VCC is present at the input E1 and, at the same time, the auxiliary supply voltage Vaux is present at the input E2, the FET transistor S1 is turned on and the FET transistor S2 is turned off, with the result that the auxiliary supply voltage Vaux is present at the output A of the interface circuit in this case.
The table below shows the various operating cases for the interface circuit according to the prior art, as is shown in FIG. 2.
TABLE 1(Prior art)Operating caseE1E2S1S2ΔB100offon0B2VCC0offonVCCB30VauxonoffVauxB4VCCVauxonoffVaux
The interface circuit according to the prior art as shown in FIG. 2 has the disadvantage, however, that it does not ensure a reliable voltage supply of the PCI plug-in card circuit in every operating case. In PCI bus systems, the main supply voltage VCC and also the auxiliary supply voltage Vaux are 3.3 volts in each case. The supply voltage of the circuit situated on the plug-in card should never be below 3 volts. Therefore, it must be ensured that, at the output A of the interface circuit, given the presence of a supply voltage on the PCI bus, likewise at least an output voltage of 3 volts is output to the PCI plug-in card K.
In operating case B3 (see table), the auxiliary supply voltage Vaux is present at the input E2, while no main supply voltage VCC is present at the input E1. In this case, the FET transistor S1 is turned on and the FET transistor S2 turns off. The FET transistor S1 acts like a forward-biased diode across which a diode forward voltage of about 0.7 volt is dropped. Given an auxiliary supply voltage of 3.3 volts, only a supply voltage of about 2.6 volts thus passes to the output A of the interface circuit according to the prior art. In other words, this supply voltage is clearly below the required 3 volts which are necessary for the reliable voltage supply of the circuit on the PCI plug-in card.